Human immunodeficiency virus or HIV is a retrovirus that causes Acquired Immunodeficiency Syndrome (AIDS), a condition in which the immune system begins to fail, leading to life-threatening opportunistic infections.
HIV was classified as a member of the genus lentivirus, part of the family of retroviridae. Lentiviruses are transmitted as single-stranded, positive-sense, enveloped RNA viruses. Upon entry of the target cell, the viral RNA genome is converted to double-stranded DNA by a virally encoded reverse transcriptase that is present in the virus particle. This viral DNA is then integrated into the cellular DNA by a virally encoded integrase so that the genome can be transcribed. Once the virus has infected the cell, two pathways are possible: either the virus becomes latent and the infected cell continues to function, or the virus becomes active and replicates, and a large number of virus particles are liberated that can then infect other cells.
HIV primarily infects vital cells in the human immune system such as helper T cells (specifically CD4+ T cells), macrophages and dendritic cells. HIV infection leads to low levels of CD4+ T cells through three main mechanisms: firstly, direct viral killing of infected cells; secondly, increased rates of apoptosis in infected cells; and thirdly, killing of infected CD4+ T cells by CD8 cytotoxic lymphocytes that recognize infected cells. When CD4+ T cell numbers decline below a critical level, cell-mediated immunity is lost, and the body becomes progressively more susceptible to opportunistic infections. Eventually, most HIV-infected individuals develop AIDS and die; however about one in ten remain healthy for many years, with no noticeable symptoms.
HIV infection in humans is now pandemic. As of January 2006, the Joint United Nations Program on HIV/AIDS (UNAIDS) and the World Health Organization (WHO) estimate that AIDS has killed more than 25 million people since it was first recognized on Dec. 1, 1981, making it one of the most destructive pandemics in recorded history. Furthermore, many people are unaware that they are infected with HIV, and elevate the risk to transmission of this disease. Since donors may therefore be unaware of their infection, donor blood and blood products used in medicine and medical research are routinely screened for HIV.
Another threatening disease, Hepatitis B is a disease of the liver caused by the Hepatitis B virus (HBV), a member of the Hepadnavirus family and one of several unrelated viral species which cause viral hepatitis. It was originally known as “serum hepatitis” and has caused current epidemics in parts of Asia and Africa. HBV is a non-cytopathic virus. This means that the virus, itself, does not cause direct damage to liver cells. Instead, it is the immune system's aggressive response to the virus that usually leads to inflammation and damage to the liver (hepatitis). However, HBV can cause damage to the genetic material inside liver cells. This can lead to liver cancer which, like hepatitis, can also be fatal.
HBV is very similar to HIV in the ways it is transmitted: through direct blood-to-blood contact and through sexual activity. However, blood levels of HBV are much higher than for HIV, making this virus much easier to transmit in certain situations (e.g., from mother to child during delivery).
Like HBV, Hepatitis C virus (HCV) infection causes liver inflammation. The parenteral route of infection seems to be most prevalent, with high rates of infection seen in intravenous drug abusers, haemophiliacs and recipients of unscreened blood transfusions. It has been estimated that 2.2% of the world's population, ˜130 million people, are infected with hepatitis C.
HCV is a small (50 nm in size), enveloped, single-stranded, positive sense RNA virus in the family Flaviviridae. HCV mainly replicates within hepatocytes in the liver, although there is controversial evidence for replication in lymphocytes or monocytes. Circulating HCV particles bind to receptors on the surfaces of hepatocytes and subsequently enter the cells. Two putative HCV receptors are CD81 and human scavenger receptor class B1 (SR-BI). However, these receptors are found throughout the body. The identification of hepatocyte-specific cofactors that determine observed HCV liver tropism is currently under investigation
As with HBV, chronic HCV infection is a major risk factor for HCC. >80% of the world's 530000 cases of liver cancer per year are caused by viral hepatitis infection, with ˜60% associated with HBV and ˜40% with HCV. Time from HCV transmission to development of cancer ranges from 10 to 50 years (median 30 years). There is a strong association between chronic HCV infection, cirrhosis, and hepato-carcinogenesis
Thus, to prevent disease transmission and diagnosis of HIV, HBV, and HCV, a good way to specifically and quickly detect and identify those viruses is recommended. In most cases, virus identifying consists of initial screening with an enzyme-linked immunosorbent assay (ELISA) to detect antibodies to virus protein. Specimens with a non-reactive result from the initial ELISA are considered negative unless new exposure to an infected partner or partner of unknown infection status has occurred. Specimens with a reactive ELISA result are retested in duplicate. If the result of either duplicate test is reactive, the specimen is reported as repeatedly reactive and undergoes confirmatory testing with a more specific supplemental test (e.g., Western blot or, less commonly, an immunofluorescence assay (IFA)). Only specimens that are repeatedly reactive by ELISA and positive by IFA or reactive by Western blot are considered positive and indicative of virus infection. Specimens that are repeatedly ELISA-reactive occasionally provide an indeterminate Western blot result, which may be either an incomplete antibody response to virus in an infected person or nonspecific reactions in an uninfected person. Although IFA can be used confirm infection in these ambiguous cases, this assay is not widely used. Generally, a second specimen should be collected >1 month later and retested for persons with indeterminate Western blot results. In addition, a few tested specimens might provide inconclusive results because of a low quantity specimen. In these situations, a second specimen is collected and tested for virus infection.
Although less commonly available, nucleic acid testing (e.g., viral RNA or proviral DNA amplification method) can also help diagnosis in certain situations and become more acceptable. The importance of nucleic acid testing (NAT) has become increasingly evident during the last decade for many purposes, such as diagnosing viral infections, monitoring antiviral therapy, and improving the safety of blood supplies. NAT combines the advantages of direct and highly sequence-specific detection of the genome of an infectious agent with an analytic sensitivity that is several orders of magnitude greater than that of antigen detection or virus isolation methods. NAT also reduces the risks of viral transmission during the period between infection and seroconversion, of infection with immunovariant viruses, of immunosilent carriage, and of occult carriage. For example, the risky period of HBV detecting by ELISA-based assay is 45-55 days, HCV is 72 days, and HIV is 22 days. However, when using NAT, the risky period of HBV can be shortened to 35-45 days, HCV can be shorten to 13 days, and HIV can be shortened to 11 days. NAT theoretically cannot eliminate the infection risky period completely, but its application can practically reduce the dangerous transmission of infectious disease.
Unfortunately, the investment cost for instruments, a running cost for reagents, a lack of maintenance support, complex and often labor-intensive procedures that require trained personnel, and the need for cold-chain transport and storage of reagents render NAT unaffordable in the settings where it is needed most. These settings include predominantly the resource-limited countries of Africa, Asia, and Latin America with high prevalence of infectious diseases. Consequently, two complementary approaches to lower the costs of NAT have been proposed: pool testing for large-scale screening and the development of multiplex assays for the simultaneous detection of several infectious agents, as refer to Ohnuma H. et al, Microbiology Immunol.45, p667 (2001) and Defoort J.-P et al, J. Clinical Microbiology. 38, p1066 (2000).